A spark plug includes a ground electrode and a center electrode disposed to face each other with a gap (clearance) therebetween, and generates spark discharge upon application of high voltage between the two electrodes. However, it is known that, since spark discharge causes an instantaneous change in current, radio noise is generated at the time of ignition by the spark plug. If this radio noise becomes severe, the noise not only affects electronic devices, such as an ECU (Engine Control Unit), of a vehicle or the like onto which the spark plug is mounted, but may also exert electromagnetic interference on the surroundings. Heretofore, various techniques have been proposed in order to reduce such radio noise. (See, for example, Japanese Patent Application Laid-Open (kokai) No. S61-135079).
However, since radio noise generated in a spark plug includes radio waves in a wide frequency range from a low frequency to a high frequency, in actuality, conventional techniques, including the above-mentioned prior art technique, cannot reduce the radio noise sufficiently.